The recent growth in information services and data processing industries has produced a need for increased speed and capacity of storage systems that manage and store large amounts of data. Companies (and consumers in general) spend a considerable amount of money to continually upgrade their systems with faster processors, more storage, and more memory; we see evidence of it everyday in desktop computers.
The same desire to increase speed and capacity exists in both the server and storage markets as well. For storage devices, and the storage market in general, increased capacity comes in the form of higher bit densities. Increased speed comes in the form of faster hard-disk drives supporting greater and yet greater rotational speeds to reduce data access times. Notably, this increase in speed can not continue indefinitely because hard-disk drives are electromechanical devices with mechanical limitations as well as electrical limitations.
One solution for decreasing information access times is to replace hard-disk drive devices with solid-state memory devices. Solid-state devices by their very nature are more reliable because they do not wear out from friction, lack of lubrication, and other technical deficiencies that plague devices that include moving parts.
The semiconductor industry has been working hard to increase the speed of semiconductors as well as improve their density. One example of this is the semiconductor memory chip such as RAM (Random Access Memory). Today, there are several types of RAM, including Synchronous Dynamic RAM (SDRAM), Dual Data Rate Dynamic RAM (DDRDRAM), as well as yet other types of RAM devices. The storage density of these devices has historically doubled about every 18 months, with no apparent end in sight, to the point where it is now possible to store a billion bits (i.e., one gigabit) of information in a single silicon memory chip.
In addition to increased density, there have also been improvements in the speed of semiconductor devices. For example, just a couple of years ago, RAM was capable of operating at 66 MHz. Today, a standard speed of RAM devices is 200 MHz and 266 MHz.
The storage industry has experienced similar improvements in memory density. For example, capacities of hard-disk drives have doubled every 8 to 10 months. This density improvement has been driven (at least in part) by increased processor speeds, expansion of database sizes, and an ever increasing desire to instantly access data.
Expansion of storage capacity has created some new applications such as Storage Area Networks (SAN), Network Attached Storage (NAS) and streaming media applications. A SAN is similar to a Local Area Network (LAN), except that it is typically used to attach multiple servers to multiple storage devices. It may best be described as a LAN for Storage.
SANs are quite useful in applications in which multiple servers need to access the same stored data. By connecting a group of servers to a group of storage devices, the storage and data can be shared. However, when attaching multiple servers to a shared group of storage devices, performance issues may arise due to contention for a particular storage device. For instance, if more than one server tries to access the same storage device, the storage device can communicate with only one server at any instant in time, during which other servers will need to wait. Wait times are typically minimal when there are few servers trying to access a common storage device. However, as more and more servers are added to a SAN, storage contention typically becomes increasingly problematic.
To help alleviate the contention problem as well as improve scaling and redundancy, the storage industry has implemented solutions such as RAID (Redundant Array of Independent Disks), disk drives with controller caching, RAID controller caching, data striping, multiple controller interfaces, etc. However, a remaining problem is that all of these solutions are still limited by the actual transfer rate and latency of the hard-disk drive itself, which is the fundamental building block of many existing storage systems.
Disk drive transfer rates limit the amount of data that can be transferred during in a given interval of time. The disk drive transfer rate is derived from the bit density on the disk platter itself as well as the rotational speed of the disk drive. As the storage capacities increase, the transfer rates also increase.
Latency, or access delay, is a measurement of time between requesting a particular segment of data and a time when the disk drive can actually transfer the requested data. A latency metric is largely derived from the seek-time of the disk-drive, which is the time it takes to position the read/write heads to the required location on the disk. While the storage density and capacity of disk drives have been increasing dramatically, the seek-time and latency have not improved at the same pace.